A serial bit channel is an information channel that carries digital information as a sequential series of ones ("1") and zeros ("0"). Since the bits are transmitted sequentially, only one conductor is required to implement a serial bit channel. (However, most implementations use another conductor for ground/reference and may use additional conductors for flow control, timing, etc.)
One simple method of implementing a serial line is to designate a ground voltage value as a clear ("0") bit and a +5 voltage value as a set ("1") bit value. In such a system, a long series of set ("1") bits will cause the physical media of the serial channel to have a positive direct current (DC) bias. Similarly, a long series of clear ("0") bits will cause the serial channel to become biased towards ground. This biasing in either direction may eventually cause data interpretation errors.
To prevent such data interpretation errors, it would be desirable to limit the disparity between the number of set ("1") bits transmitted and the number of clear ("0") bits transmitted.